1. Field of the Invention
The present invention relates to a motor capable of generating mechanical power without relying on an electro-magnetic force, and more particularly to a compact and light-weight motor of such a type suitable for use in compact portable equipment such as camera.
2. Related Background Art
Among motors in the general sense, or devices for generating rotary power there are already known various principles of power generation, but motors in the narrow sense almost always refer to electric motors generating mechanical rotary power by an electromagnetic force, as they are most compact and most practical for use.
As recent technical developments in electric motors have enabled significant compactization, small motors are widely employed in cameras and other portable equipment, thus contributing to the improvement of the performance of such equipment.
However, compact motors employed in cameras or the like are capable of high-speed rotation but are incapable of generating a large torque at a low rotation speed. For this reason there is required a reduction mechanism, and the power generating mechanism including the motor and the reduction mechanism inevitably becomes considerably large. Therefore the development of a compact, light-weight motor capable of generating a large torque at a low rotation speed without the reduction mechanism has been needed.
In view of such background, recent trials in utilizing piezoelectric elements as a power source has led to the development of an ultrasonic motor in which an ultrasonic vibration generated by piezoelectric elements is converted into a rotary motion.
Such a known ultrasonic motor, for example, disclosed in U.S. Pat. Nos. 4,580,073 and 4,587,452, has a circular stator and a circular rotor, and a piezoelectric element adhered circularly to an end face of the circular stator which generates a circular elastic bending wave on the end face of the stator, thereby rotating the rotor maintained in contact with the end face of the stator.
Such a known ultrasonic motor, being capable of generating a large torque at a low rotation speed, can dispense with the reduction mechanism, and is therefore suitable for use in small equipment such as a camera, and is particularly suitable as an automatic focusing motor in a camera, because of its annular form. However in such a known ultrasonic motor, it has been difficult to support the stator in such a manner as to prevent attenuation of vibration. The support of the stator is particularly difficult in equipment susceptible to external shock, as in the case of a camera.
On the other hand, Japanese Patent Laid-open No. 55012/1986, corresponding to U.S. Pat. No. 4,763,776 proposes a technology of adhering electromechanical converting elements to plural rod-shaped elastically vibrating members, and causing a rotary motion in the elastically vibrating members by the vibrations generated in the converting elements, thereby transporting an article placed on the vibrating members.
Although the technology disclosed in the abovementioned reference is effective in causing a linear motion of the article along the surface of the vibrating members, it is not suitable for causing a rotary motion in an annular article such as a lens barrel.